A reed relay is a common type of relay. The reed relay includes one or more thin cantilevered metal arms or reeds made of paramagnetic material such as permalloy (typically 80% nickel, 20% iron). In the presence of a magnetic field, the reeds experience a force and move to make contact with one another or another electrode to complete a circuit. While these relays can be used to switch DC signals for powering devices, AC signal switching applications dominate the areas of application for small reed relays. Reed relays have the ability to handle large currents, have long lifetimes, typically more than 1×108 cycles, relatively low cost, moderate contact resistance and good isolation.
However, reed relays typically have a number of drawbacks. There is typically a lower bound on the size of reed relay because of the space occupied by the winding of the electromagnetic actuator. The presence of the lower bound on the size of the reed relay typically limits switching speed and unless designed to mechanically latch, the inductive nature of the relay requires that significant power is typically required for the relay to remain latched. Electromechanical bounce issues may exist that impart noise into the switched signal along with contact wear issues for reed relays. Additionally, reed relays cannot operate at frequencies greater than about 5 GHz.
A way to improve the performance of a reed relay is to coat the electrodes with liquid mercury, thereby replacing a solid—solid contact with a liquid—liquid contact. A liquid—liquid contact provides a number of advantages by removing the electromechanical bounce issues associated with solid—solid contact; eliminating most of the contact wear issues because the liquid is refreshed every time the relay is actuated; the actuating force required to make a good contact is typically reduced; and the contact resistance and insertion loss is reduced. However, in a liquid—liquid contact the surface tension forces which need to be overcome typically tend to dominate as the relay size is scaled down, thereby setting a limit on switching speed.
MEMS (MicroElectroMechanical Systems) techniques have been introduced to improve the speed, lower the cost and provide multiple relays in a compact package, allowing reed relays to be made at sizes on the order of a few square millimeters. Reduced size allows some reed relays to be capable of operating at switching speeds greater than about 1 kHz. However, typically contact resistance is high due to the low contact forces that are possible with the typical electrostatic actuation. Some MEMS relays have higher force actuators but typically sacrifice speed and lifetime.
Some contact related limitations have been addressed by liquid metal microrelays, such as those disclosed, for example, in U.S. Patent Publication 20030201855 A1, which are latching MEMS relays that depend on a thermal actuator and all liquid contact to lower the insertion loss. Because the relay is latching, no power is required for the relay to remain actuated. The liquid metal microrelays are suitable for radio frequency (RF) signals and typically provide a bandwidth of 18 GHz. However, liquid metal microrelays have several problems. The thermal actuator cannot typically be repeatedly operated without heat buildup because the thermal actuator heats up much more quickly than it cools and this places an upper bound on how rapidly the relay may be cycled. Liquid metal microrelays are also typically sensitive to the amount of liquid mercury they contain and the volume of mercury involved is typically relatively large compared to the relay volume.